Add a response header to indicate when site is in maintenance mode. Addresses #637

Author: vvuksan

etc/vcl_snippets/deliver.vcl

@@ -40,6 +40,9 @@
     # Add an easy way to see whether custom Fastly VCL has been uploaded
     if ( req.http.Fastly-Debug ) {
         set resp.http.Fastly-Magento-VCL-Uploaded = "1.2.206";
+        if (table.lookup(magentomodule_config, "allow_super_users_during_maint", "0") == "1") {
+            set resp.http.Fastly-Magento-Maintenance-Mode = "on";
+        }
     } else {
         remove resp.http.Fastly-Module-Enabled;
         remove resp.http.fastly-page-cacheable;